The present invention relates generally to improvements in television receivers. It is particularly directed to a system for generating vertical drive pulses for use in controlling the vertical scan of a television receiver, and for generating horizontal rate gating pulses of the type used to gate AGC (automatic gain control) circuitry.
In some television receivers, a so-called vertical countdown circuit is employed to generate a vertical drive pulse after counting a predetermined number of horizontal rate clock pulses which are synchronized with incoming horizontal sync pulses. When the composite video signal is derived from a standard broadcast signal having 525 lines per frame (NTSC), the countdown circuit is put in a "standard" mode for generating vertical pulses at the conventional rate. However, when the composite video signal is derived from a non-standard signal source, such as a video camera, the number of horizontal lines per frame may be higher or lower than the standard rate. In that case, the vertical countdown circuit goes into a "non-standard" mode in order to generate vertical drive pulses at a rate which synchronizes the receiver's raster with the non-standard signal source.
Typically, when a non-standard composite video signal is received, the vertical drive pulses developed by the vertical countdown circuit have a relatively short duration which can vary from field to field. When a standard video signal is received, vertical drive pulses of a relatively longer and more constant duration are generated. Because good interlace is difficult to achieve with vertical drive pulses whose widths are changing, the latter pulses are commonly applied to a monostable multivibrator to convert them to vertical pulses of a relatively constant width.
The problem with the arrangement described immediately above is that the vertical countdown circuit is normally constructed on an integrated circuit chip, and the monostable multivibrator requires an extra pin on the chip to access a discrete RC network which controls the duration of the multivibrator's output pulses. In addition to the expense associated with the extra pin and the discrete RC network, the RC network also includes the usual component tolerances which result in a corresponding, undesirable tolerance in the duration of the multivibrator's output pulses.
To provide vertical drive pulses of a more constant width, digital circuitry whose timing is more accurate and consistent than conventional passive circuitry can be used. However, the inclusion of additional, relatively complex digital circuitry is difficult to justify from an economic point of view unless such circuitry can also be used to provide other functions in addition to the generation of constant width vertical drive pulses.
The present invention not only provides the requisite constant width vertical drive pulses with digital circuitry, but also time-shares portions of its circuitry to generate horizontal rate gating pulses for use in gating AGC circuitry, burst detection circuitry and the like.